The containers made of tin or multicolored sheet metal, glass or else ceramic, common in the past, are increasingly being replaced by containers made of plastic. Primarily plastic containers are now used for the packaging of fluid substances, for example beverages, free-flowing foods such as, e.g., ketchup, sugo, pesto, sauces, mustard, mayonnaise, and the like, household products, care products, cosmetics, etc. The low weight and the lower costs certainly play a significant role in this substitution. The use of recyclable plastic materials, the use of bioplastics, and the overall more advantageous total energy balance in their production also contribute to promoting the acceptance of plastic containers, in particular plastic bottles, by consumers.
A large number of the plastic bottles and similar plastic containers now used are produced in a stretch blow-molding method. With this method, first a so-called preform with a usually elongated, tube-like shape is produced, which preform is closed with a bottom on one of its longitudinal ends and has a neck section with means for positive clamping of a closure part equipped with corresponding engagement means on the other longitudinal end. The means for positive clamping of a closure part can be, for example, threaded sections made on the outside wall of the neck part or bayonet-like projections or corresponding recesses.
The production of the preform can be carried out in an injection-molding method. However, alternative production methods for preforms are also known, for example impact extrusion or extrusion blow molding. The production of the preforms can be done separated in time and/or space from the subsequent stretch blow-molding method. In an alternative method, the preform that is produced is further processed immediately after its production without interim cooling. For stretch blow-molding, the preform is inserted into a mold cavity of a blow mold and can be inflated by a fluid, usually air, which is introduced with overpressure, expanded in the radial and axial directions. In this case, the preform is stretched in the axial direction in addition with an elongated mandrel that is run-in through the neck opening of the preform. After the elongation/blow-molding process, the finished plastic container is demolded from the blow mold.
In various applications, the plastic containers, for example for reasons of sterility or to improve the flow properties of the filling material, are bottled hot or at least warm. In this case, a hot bottling can be defined as a bottling of the filling material at a temperature of for example, 60° C. to 100° C. After the bottling, the containers are tightly closed, for example with a sealing membrane, and tilted in order to moisten the membrane. Upon cooling the filling material and the headspace, in the case of a reaction of the filling material with gases in the head space, a vacuum can be formed inside the container by a phase shift of gases (vapor to water) or else in the case of more intensive dissolving of gases with the filling material, which can have the result that the container becomes deformed. The deformation is a result of the pressure difference between the atmospheric pressure that acts on the container walls from the outside and the underpressure that develops in the interior because of cooling or the above-named effects.
In contrast, shortly after the bottling, an overpressure can also develop, for example, by the evaporation of the liquid, by degassing of the product, or by an elevated temperature of the filling material in the container neck or in the headspace of the container. After the container is cooled, however, an underpressure can result, since the gases in the headspace and the filling material itself greatly change their volumes, in particular decrease, because of the temperature difference. Underpressure can also develop in that a portion of the gas contained in the headspace is dissolved in the filling material or reacts chemically with the latter. For example, vitamin-C-containing products or products containing unsaturated fatty acid can react with the oxygen in the headspace and thus can result in underpressure.
Also, it may happen that certain contents leave the plastic container by migration processes through the container wall and leave behind an underpressure. Thus, for example, the water loss in the case of PET containers can be one-half to one percent after only one year. In the case of especially thin-walled PET bottles or in the case of PET bottles that are stored at a high temperature, this loss can be even still higher. In the case of other materials, such as, e.g., in the case of PLA, polystyrene, a water loss in the indicated order of magnitude can also occur even after a much shorter time. To some extent, a volume contraction can even occur because of a cooler environment (refrigerator), a cooler time of year (winter), or specific geographic latitudes (cold regions).
Also, the bottling and marketing of plastic bottles at different elevations above sea level can lead to deformations. Thus, in the case of plastic bottles that are bottled, for example, at the elevation of Mexico City and then are transported to the coast and distributed there, the risk of a deformation because of the greater air pressure prevailing on the coast exists. Specifically, this deformation of the plastic container in general has no effect on the quality of the filling material. For the consumer, however, often the outer manifestation of an article for sale is decisive for its selection. A plastic container that has deformations can therefore often result in the erroneous assumption of the customer that the product contained in the container no longer has the desired quality properties.
This tendency of warm or hot bottled filling material to deform during cooling can be counteracted specifically by making the wall thickness of the plastic container greater. Because of the increased need for material, however, the production of this plastic container becomes more expensive and its weight increases. Solutions are also known in which along the axial extension of the plastic container, fins and grooves are provided as stiffeners. In most cases, the mechanical stiffeners involve specially formed extrusion nozzles or injection nozzles for the production of the preforms and a special process control. Plastic bottles that have deformable areas distributed over their periphery, which areas are to compensate the pressure differences, are also known. In turn, however, these deformable areas are an obstacle to applying labels to the bottle surfaces. Also, plastic containers with flexible bottoms, which are to compensate the pressure differences, are already known. Apart from the fact that the production of such flexible bottoms is very labor-intensive and costly and thus only relatively small pressure differences can be compensated, a more flexible bottom can lead to problems with the stability of the container.
In contrast, however, it can also result in an increase in volume of the filling material, for example in the case of pasteurization, in which the filled and closed plastic container undergoes, for example, a hot shower. This increase in volume can result in a bulge in the container body or even in a turning-outward of the container bottom, which can impair the stability of the container. Both effects, which are highly undesirable, can be counteracted by a correspondingly increased wall thickness in the container body and/or the container bottom. Because of the increased need for material, however, the production of this plastic container becomes more expensive and its weight increases.